The aim of the project is to investigate isolates of feline leukemia virus (FeLV) that induce a rapid and fatal disease of the hemopoietic system, erythroid hypoplasia. This work may provide a model of general relevance for degenerative diseases induced by retroviruses, including those of man, and could increase our understanding of the genesis of pure red cell aplasia in humans. The viruses to be studied are of FeLV subgroup C. A number of these relatively rare viruses have been collected and will be compared by restriction enzyme mapping, heteroduplex analysis and DNA sequence analysis. The viral sequences necessary for the FeLV-C disease will be investigated by constructing hybrid viruses from standard FeLV-A and FeLV-C strains, in which portions of the genome have been exchanged. This should allow identification of viral determinants of pathogenicity. The cellular and molecular mechanisms involved in erythroid hypoplasia induced by FeLV will be investigated by in vitro assay of the effect of intact viruses, viral genes or purified gene products on fractionated cat bone marrow cells.